Under "Permissions" section, you can see "Block pop-up windows." Uncheck this option if you want to enable popup.

That is all. The popup is enabled.

Note:

You can allow popup for certain websites alone without enabling popup and here is how:

Click on "Exceptions" button, and you can see a popup. Type the website URL for which you want to allow popup in the "Address of website" field and then click "Allow" button. Repeat the previous step and add the websites you want. After that, click on "Save Changes" button.